RIVER FALLS, WIS. - Sometimes, it takes years to see the impact of a Supreme Court decision on American life, and sometimes a ruling lands with an explosion.

The Roberts Court's game-changing decisions on campaign finance reform have been both.

Almost from the moment Chief Justice John G. Roberts Jr. joined the bench five years ago, the court's conservatives have acted systematically on their deep skepticism of campaign spending restrictions. They have repeatedly questioned the ability of Congress to regulate the role of wealth and special interest involvement in elections without offending the First Amendment guarantee of unfettered political speech.

The court's rulings are being felt this year everywhere voters go to the polls. But they have special resonance in Wisconsin, where Sen. Russ Feingold, the Democratic author of the McCain-Feingold campaign finance reform act, has seen not only his legislative legacy but his Senate career endangered.

"I've always been a target in this stuff," Feingold said during a recent swing through the western part of his state. "And this year, I'm getting the full dose: over $2 million in these ads [criticizing him] that used to not be legal."

The court's rulings on campaign finance have assured it the most prominent role in the country's elections since its polarizing decision 10 years ago in Bush v. Gore. They also provide perhaps the most striking example of how the Roberts Court differs from its predecessor.

In decision after decision, a slim majority of the court has cut back major parts of McCain-Feingold and other campaign-spending restrictions. The capstone came in January, with its 5 to 4 decision in Citizens United v. Federal Election Commission that rewrote decades of law and said corporations and unions could spend unlimited amounts to support or oppose candidates.

The court has declined two opportunities to expand on the Citizens United ruling, however. On Monday, the court declined to hear a case that sought to loosen the disclosure requirements on groups that spend money independent of candidates. Last summer, it chose not to hear a Republican challenge of limits placed on contributions to political parties.

It's not surprising, Feingold told a small group of supporters recently on the windswept campus of the University of Wisconsin at River Falls, that outside groups are gunning for him. He pointed to a recent piece in Washingtonian magazine that divided the Senate into categories.

"I wasn't fourth, third, second: I was the number one enemy of Washington lobbyists," he said, adding, "A Senate seat can't be bought; it has to be earned. We will never let the special interests drown out the voices of the people."

Polls show Feingold in serious trouble in his reelection fight with Republican businessman Ron Johnson. Johnson has invested more than $8 million of his money in the race, and although the two campaigns are competitive with each other financially, outside groups have spent nearly $3 million on Johnson's behalf.

Feingold said he has requested that outside groups stay out of the race, and a Washington Post analysis shows 92 percent of the outside spending has supported the Republican. The impact has been obvious: The Wesleyan Media Project said there have been more commercials about the Senate race in Wisconsin than in any state outside Nevada, where Senate Majority Leader Harry M. Reid (D) is running for reelection.